Backport of [OSS] Fix initial_fetch_timeout to wait for all xDS resources into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #18024 to be assessed for backporting due to the inclusion of the label backport/1.15.

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The person who merged in the original PR is:
@DanStough
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The below text is copied from the body of the original PR.

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Description

Attempt 2 at closing #17283

Because we use the default initial_fetch_timeout in the Envoy bootstrap, not all Envoy resources may be loaded in busy systems. However, we previously relied on the timeout to start ingress and API gateways. This PR:

1. Changes the default initial_fetch_timeout to wait indefinitely "0s".
2. Changes the API and Ingress Gateway code to send empty xDS as long as watches return an empty response.

Links

Equivalent Dataplane PR

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Overview of commits

• 8a2f60d - e17e53c - d919d55

[hashicorp-cla]

All committers have signed the CLA.

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR